1. Field of Invention
The present invention relates to an optical proximity correction method. More particularly, the present invention relates to an optical proximity correction method of fabricating photomasks for metallic interconnects.
2. Description of Related Art
Due to the rapid development of integrated circuit techniques, device miniaturization and integration is a common trend. In the process of fabricating an integrated circuit, photolithographic and etching processes have major influences on the ultimate operating characteristics of the fabricated devices. As the level of integration continues to increase, device dimensions as well as distance of separation between neighboring devices are reduced correspondingly. Consequently, a pattern transferred to a silicon chip using a photolithographic process may result in some deviation. For example, sharp corners in the pattern may be rounded, line-ends of a line pattern may be pulled up and line width may be reduced or increased. All these occurrences are often referred to as optical proximity effects (OPE).
The aforementioned deviations have little negative effect on the integrated circuit if dimensions of devices are relatively large or the level of integration is low. However, the negative effects on a highly integrated circuit can be serious. This is because the distance between neighboring devices in a highly integrated circuit is very small. If there is a deviated expansion in the line-width when a pattern is transferred to a silicon chip, partial pattern overlap may occur, leading to an open. In other words, any reduction of device dimensions for increasing operating speed is likely to be limited by pattern transfer fidelity in photolithographic process.
There are several possible factors leading to optical proximity effects. The main factors include optical interference during the passage of light through the pattern on a photomask, baking temperature/time during photoresist preparation, photoresist development, light reflected from an uneven substrate and the effects of etching. As dimension tolerances are reduced due to a higher level of integration, pattern transfer deviation due to optical proximity effects is likely to increase.
To prevent variation of critical dimensions in mask pattern transfer, an optical proximity correction procedure is often applied during photomask fabrication. That is, a pattern to be transferred to the semiconductor substrate of a silicon chip is first processed by computer software to obtain data for optical proximity correction of the mask pattern. The data is then filed in a computer. A patterned photomask is subsequently fabricated according to the optical proximity corrected data in the computer. Hence, when a beam of light shines on the pattern-corrected photomask, the desired pattern is formed on the semiconductor substrate.
In general, optical proximity correction includes the addition of serifs to the corners or hammerheads to end edges of the original pattern to prevent the rounding of right-angled corners and the pulling up of line-ends.
However, as the level of integration continues to increase and device dimensions continue to reduce, distances between neighboring devices shrink. In other words, tighter design rules are required. When critical dimension of the main pattern and separation between neighboring patterns are reduced to a certain extent, the addition of serifs to corners or the addition of hammerheads to ends can no longer prevent the pull-up of line-ends.
FIG. 1A is a sketch showing the addition of serifs in a conventional optical proximity correction method. As shown in FIG. 1A, a main pattern 100 (shown in dashed lines) waiting to be transferred is provided. Main pattern 100 includes a horizontal strip 100a and a vertical strip 100b (only a portion of the vertical strip is shown). Serifs are added to the corners of main pattern 100. For example, serifs 110 are added to the corners of horizontal strip 100a and vertical strip 100b. 
FIG. 1B is a magnified sketch of the tip portion of the vertical strip shown in FIG. 1A. FIG. 1B illustrates conventional dimensions and positions of those serifs 110 added to the corners of main pattern 100. For example, length 111 of the corner serif 110 is about 100 to 150 nm and width 112 of the corner serif 110 is about 100 to 150 nm. Furthermore, the corner serif 110 overlaps with the corner of a main pattern by a length 113 (about 50 nm) and a width 114 (about 50 nm).
However, when the light source is deep-ultraviolet (wavelength 248 nm) and the critical dimension of a main pattern is reduced to about 0.18 xcexcm while the distance between neighboring lines, such as the distance between horizontal strip 100a and vertical strip 100b, is smaller than 0.2 xcexcm, addition of serifs to the corners of the main pattern can no longer prevent the pull up of line-ends. This can be observed in FIG. 1A where the solid lines represent the resulting pattern after a photo-exposure while the dashed lines represent the original main pattern 100.
In general, addition of corner serifs and hammerhead patterns to a photomask is capable of reducing edge or corner variation after a pattern transfer. However, when the critical dimension or the distance between neighboring lines has been reduced to a certain extent, addition of serifs or hammerhead patterns according to conventional dimensions and ratios can no longer avoid the undesirable pull up of line-ends effectively.
Accordingly, one object of the present invention is to provide an optical proximity correction method that uses additional corner serifs or hammerhead patterns to correct and avoid pull up of line-ends in a main pattern. Furthermore, the dimensions and ratios of these corner serifs are set such that the main pattern is corrected with only slight expansion of line-end width and line-end approaching the original design in length. A second object of this invention is to provide an optical proximity correction method suitable in the design of a photomask for fabricating metallic interconnects. The invention is able to correct the main pattern with only a slight expansion of width near its end so that the problem of an end not approaching the original design after a photo-exposure is remedied. Hence, any misalignment that may lead to uncompleted contact or an open of the metallic interconnect can be avoided. Furthermore, the slightly expanded line-end permits a higher process window in the fabrication of metallic interconnects.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an optical proximity correction method. First, a main pattern waiting to be transferred is provided. A first assist feature is added to various corners of the main pattern. Distance between lines in the main pattern after placing the first assist feature is checked. If distances between neighboring lines are too small, resulting in one of the first assist features touching or overlapping with a neighboring one, a second assist feature is used to replace each of the conflicting first assist features.
This invention provides an optical proximity correction method. Assist features with different dimensions are added to a main pattern that needs to be transferred. Hence, any profile variations near the edges or corners of the main pattern after pattern transfer are reduced and processing window for the main pattern is increased.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.